He suddenly went cold

I´m meeting one guy in work (he´s from another company, we don´t work together, we just simply meet from time to time). Some time ago we had a very good conversation and I´ve got an impression he is flirting with me. As I´m very shy I probably was not reacting and didn´t show signs of interest. In the meantime I was two weeks on holiday. When I came back to work , I decided that I want to move this forward, so I found an excuse to email him (related to work stuff), but he responded only with shortly "yeap, thanks".. nothing else. Why is he suddenly so cold after he showed he is interested? Please help.